1. Field of the Invention
The present invention relates to a method for producing a magnetic recording medium such as a digital audio tape (DAT), a digital video tape (DVT), a 1/2 inch video tape, an 8 mm video tape and the like. More particularly, it relates to a method for producing a magnetic recording medium having decreased drop out and increased electromagnetic characteristics.
2. Description of the Prior Art
Magnetic recording medium is increasingly required to have high density recording characteristics. To satisfy such a requirement, it is important to decrease loss during recording and reproducing in a short wavelength range. However, when recording and reproducing in the short wavelength range, drop out is caused by minute protrusions on a magnetic layer of the magnetic recording medium and debris, which is powder drop from a surface of the magnetic recording medium even when the size of protrusions or the amount, of debris is so small as to not cause serious drop out in the conventional recording wavelength range.
Hitherto, to prevent the drop out, it has been proposed to increase the dispensability of magnetic paint by improving the dispersing conditions under which the paint is applied or the apparatus for applying the magnetic paint, or by modifying the magnetic paint composition, or by improving calendering conditions after the magnetic paint is disposed on a substrate. However, the drop out in the short wavelength range cannot be prevented by such conventional techniques.
In the production of a floppy disc, it is known from U.S. Pat. No. 4,499,122, to polish a surface of the magnetic layer thereof, and recently incorporating a polishing step in a method for producing magnetic recording tape has been investigated.
Since the magnetic tape medium is required to have increased high density recording characteristics, the medium including 8 mm video tape, DAT and DVT, a minute degree of debris causes great drop out. To prevent such powder drop, the surface of the magnetic layer of the medium is polished. Although the polishing can decrease debris, to achieve a satisfactory decrease of drop out, polishing conditions should be intensified or the number of polishing steps should be increased. However, the intensification of polishing or an increase in the number of times the medium is polished will damage the surface of magnetic layer and in turn induce deterioration of electromagnetic characteristics of the magnetic recording medium. In an extreme case, the drop out increases.